The present invention is with respect to an apparatus for measuring reflectance at a reflecting surface placed in a field of measurement, using a transmitter and receiver.
Apparatus on these lines has been designed for measuring reflectance at light reflecting surfaces, for measuring the reflectance values and for detecting and measuring changes in reflectance quantitatively. In known apparatus using only one transmitter illuminating the field of measurement with only one single beam, for example at an angle of 45.degree., the lengths of the paths of the light from the light source to the different points in the field of measurement are different in length. Because the intensity of illumination is a function of the square of the path of the illuminating light, the field of measurement is not evenly illuminated so that there will in fact be different intensities of illumination at the different single points in the field of measurement. It is because of this inhomogeneous illumination of the field of measurement that different readings will be produced for the reflecting points in the field of measurement so that the measurements and readings for the reflectance will not be true. Seeing that the separate points of the field of measurement have to be acted upon by a given intensity of radiation, relatively high-intensity sources of radiation are needed that have an undesired heating or thermal effect on the sample. To put an end to such overgreat thermal effects on the sample, the radiating light source has to be kept at some distance from the field of measurement and may not be placed at any desired short distance therefrom so that the reflectance measuring apparatus has to be of a certain size and for this reason the degrees of freedom when designing the measuring apparatus are limited.